Enhanced solder joint strength and ease of inspection of leadless leadframe package (LLP)

ABSTRACT

Methods of fabricating leadless packages are described that provide good solder joint reliability. In most respects, the packages are fabricated in a manner similar to current lead frame based leadless packaging techniques. However, at some point in the process, the contacts are provided with undercut regions that are left exposed during solder plating so that the solder plating also covers the exposed side and undercut segments of the contacts. When the resultant devices are soldered to an appropriate substrate (after singulation), each resulting solder joint includes a fillet that adheres very well to the undercut portion of contact. This provides a high quality solder joint that can be visually inspected from the side of the package.

BACKGROUND OF THE INVENTION

The present invention relates generally to the packaging of integratedcircuits. More particularly, the invention relates to leadless packagingdesigns and processes.

A leadless lead frame package (LLP) is an integrated circuit packagedesign that contemplates the use of a lead frame in the formation of achip scale package (CSP). The resulting packages are sometimes referredto as quad flat packs—no lead (QFN) packages. As illustrated in FIGS.1(A)-1(C), in typical leadless lead frame packages, a copper lead framestrip or panel 101 is patterned (typically by stamping or etching) todefine a plurality of arrays 103 of device areas 105. Each device area105 includes a die attach pad 107 and a plurality of contacts 109disposed about their associated die attach pad 107. Very fine tie barsare often used to support the die attach pads 107 and contacts 109. Thecontacts 109 are generally attached to the tie bars 111 by tie bar stubs112.

During assembly, dice are attached to the respective die attach pads 107and conventional wire bonding is used to electrically couple bond padson each die to their associated contacts 109 on the lead frame strip101. After the wire bonding, a plastic cap is molded over the topsurface of the array 103 of wire-bonded dice. The dice are generallythen singulated and tested using conventional sawing and testingtechniques. One assembly process is illustrated graphically in steps160-170 of FIG. 6(A).

FIGS. 2A and 2B illustrate a segment of a molded lead frame panel priorto singulation. The die attach pad 107 supports a die 120 which iselectrically connected to its associated contacts 109 by bonding wires122. A plastic casing 125 encapsulates the die 120 and bonding wires 122and fills the gaps between the die attach pad 107 and the contacts 109,thereby serving to hold the contacts in place. Once the plastic casing125 has cured, the bottom surfaces of the contacts 109 and the dieattach pad 107 are buffed and solder-plated prior to singulation. Thesolder plating may form a thin solder layer 140 on the bottom surface ofthe die attach pads and contacts.

FIG. 2C illustrates a sawing-based singulation (or dicing) process. Asshown therein, a saw blade 130 is directed along the tie bar axis,thereby severing the tie bar 111 (and removing corresponding portions ofthe molding material 125 and often a small portion of the tie bar stubs112) as it proceeds. Once the tie bar 111 has been severed, only themolding material 125 holds the contacts 109 in place. The process isrepeated along each tie bar in the two-dimensional matrix of tie bars,with a single pass being used to cut along each tie bar. When the dicingis complete, the resulting packaged chip can then be surface-mounted ona printed circuit board or other substrate using conventionaltechniques, such as soldering, as generally illustrated in FIG. 5A. Asseen therein, compact solder joints 150 are typically formed between thepackage and the corresponding attach pads 151.

Since leadless lead frame packaging have proven to be a cost effectivepackaging arrangement, there are continuing efforts to provide furtherimprovements to the package structure and/or processing to permit thepackage style to be used in additional applications and/or to improvespecific characteristics of the resultant devices.

SUMMARY OF THE INVENTION

To achieve the foregoing and other objects of the invention, methods offabricating leadless packages are described that provide improved solderjoint reliability and visibility. In most respects, the packages arefabricated in a manner similar to current lead frame based leadlesspackaging techniques. By way of example, a lead frame panel may bepatterned to define a plurality of device areas and a matrix of tiebars. Each device area includes a multiplicity of conductive contactsthat are attached to an associated tie bar. At some point in theprocess, the contacts are provided with undercut regions that are leftexposed during solder plating so that the solder plating covers theexposed side and undercut segments of the contacts. After the solderplating, the lead frame panels may be processed in a conventional mannerincluding singulation. When the resultant devices are soldered to anappropriate substrate, each resulting solder joint includes a filletthat adheres very well to the undercut portion of the contact. Thisprovides a high quality solder joint that can be visually inspected andtested from the side of the package.

A variety of processes are described that facilitate the production ofleadless packages having solder plated undercut regions that requirelittle modification to existing production processes. For example,during assembly, dice are attached to die attach pads or otherwisepositioned within associated device areas. The dice are thenelectrically connected to the contacts (e.g., by wire bonding). A casingis then molded or otherwise provided that encapsulates the die andconnectors while leaving the bottom surfaces of the contacts exposed. Inone aspect of the invention, after the encapsulation, portions of thecontacts adjacent to the tie bars are undercut without severing the tiebars. By way of example, undercutting can be accomplished by a partialdepth sawing operation along the tie bars. This undercutting exposessome side and underside surfaces of the contacts. The lead frame is thensolder-plated in a conventional manner. In addition to plating thebottom surfaces of the contacts, the solder plating also covers theexposed side and underside surfaces of the contacts. After the solderplating, the lead frame panels may be processed in a conventional mannerincluding singulation.

In other embodiments, the contacts are patterned to have a stub portionthat attaches to the tie bars and a base portion that includes theexposed bottom surface. The stub portions are preferably thinner andnarrower than the base portions of their associated contacts. In someembodiments, the lead frame is patterned to define at least onetwo-dimensional array of immediately adjacent device areas that areseparated only by the tie bars. One approach to undercutting thecontacts in such an arrangement contemplates first making a shallow cutalong the tie bar axes. The first cutting operation is arranged toexpose the stubs and a side portion of the base of each contact, butdoes not sever the tie bars or the stubs. After the solder plating andother desired processing is finished, the individual packages may besingulated by a second cutting operation along the tie bar axes. In somedescribed embodiments, the first cutting operation is preformed using afirst saw blade and the second cutting operation is performed using asecond saw blade that is narrower than the first blade.

In another aspect of the invention, wells are formed in the contactportions of the lead frame during the initial patterning of the leadframe. The wells are exposed on the bottom surface of the contacts buthave side walls that prevent encapsulant from filling the wells duringthe encapsulation process. Exposing the wells during initial patterningof the lead frame eliminates the need to partially cut the contactsbecause the exposed well creates an undercut region in the contactsimilar embodiments discussed above. The exposed well is solder platedprior to a singulation cut as in standard packaging procedure. When thesemiconductor package is mounted on a substrate, the well region isfilled with solder creating a fillet similar to embodiments discussedabove.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, maybest be understood by reference to the following description taken inconjunction with the accompanying drawings in which:

FIGS. 1A-1C diagrammatically illustrate a lead frame strip suitable foruse in forming leadless lead frame packages.

FIG. 2A is a diagrammatic cross sectional side view of a small sectionof the lead frame panel of FIG. 1C after encapsulation and solderplating. FIG. 2B is an enlarged view of the contact/tie bar region ofFIG. 2A. FIG. 2C diagrammatically illustrates a singulation sawingoperation on the lead frame panel of FIG. 2B.

FIG. 3 is a diagrammatic cross sectional side view of a small section ofan encapsulated lead frame panel in accordance with one embodiment ofthe present invention illustrating a partial cutting operation.

FIGS. 4A and 4B are diagrammatic cross sectional side views of the panelillustrated in FIG. 3 after the solder plating operation and during asingulation sawing operation, respectively.

FIG. 5A is a diagrammatic cross sectional side view of an existingleadless lead frame package mounted on a printed circuit board.

FIG. 5B is a diagrammatic cross sectional side view of a leadless leadframe package mounted on a printed circuit board in accordance withanother embodiment of the present invention.

FIG. 6A is a flow chart illustrating a packaging process.

FIG. 6B is a flow chart illustrating a modified packaging process inaccordance with an embodiment of the present invention.

FIG. 7A is a 3-dimensional perspective view of a contact having a wellcreated as a result of etching the contact surface in accordance with anembodiment of the present invention.

FIG. 7B is a diagrammatic top view of a contact having a well as shownin FIG. 7A.

FIG. 7C is a diagrammatic cross sectional side view of a contact havinga well as shown in FIG. 7A.

FIG. 8A is a diagrammatic top view of a preferred embodiment of acontact having an ovate well.

FIG. 8B is a diagrammatic top view of a preferred embodiment of acontact having a substantially rectangular well.

FIG. 8C is a diagrammatic top view of two contacts and their associatedwells connected by tie bar stubs to a tie bar, which is part of a leadframe panel according to another embodiment of the present invention.

FIG. 9 is a diagrammatic cross sectional side view of a small section ofthe lead frame panel in accordance with one embodiment of the presentinvention illustrating the exposed well of the contact protected fromencapsulation material.

FIG. 10 is a flow chart illustrating a modified packaging process inaccordance with another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A number of improvements to leadless package designs are describedbelow. In the following description, numerous specific details-are setforth in order to provide a thorough understanding of the presentinvention. It will be understood, however, to one skilled in the art,that the present invention may be practiced without some or all of thesespecific details. In other instances, well known process operations havenot been described in detail in order not to unnecessarily obscure thepresent invention.

As described in the background section of the application, conventionalsemiconductor packaging processes result in a package with a pluralityof solder-plated contacts exposed on the bottom surface of the package.FIG. 5A illustrates a compact solder joint 150 that results when thepackage is mounted to a circuit board attach pad 151 which in turn ispart of a printed circuit board 153. Note that in FIG. 5A a portion ofmolding material 125 remains adhered to the underside of the contact 109(and under the tie bar stub 112) after a conventional singulation cut.The tie bar stubs 112 are exposed at the peripheral edges of the packageand are substantially co-planer with the package edge 154. The exposedportion of the tie bar stub 112 does not serve as an attachment pointfor the leadless lead frame package. Thus, the solder joint 150 betweenthe contact 109 and the printed circuit board attach pad 151 is confinedto a relatively small area (i.e., the area of the contact pad).

Although the solder joint 150 of FIG. 5A works well in a wide variety ofapplication, in some applications it may be desirable to provide bettersolder joint visibility to better facilitate visual inspection of thesolder joints and/or inspection by certain types of inspection machines.A weaker, less visible solder joint results because the molding material125 partially obscures the joint. As can be appreciated by one skilledin the art, in some circumstances better visual confirmation of thesolder joint of a particular package is desirable both for qualityassurance and for troubleshooting purposes.

Referring next to FIG. 5B, a package in accordance with the presentinvention will be described. FIG. 5B illustrates a leadless lead framepackage attached to a printed circuit board 153. In this illustration,the molding material previously attached to the underside of the contact109 is removed such that the solder joint 152 fills a larger region andis readily visible from the side of the package.

One advantage of the present embodiment is that the solder joint 152 maybe more easily inspected. Whereas the molding material 125 in FIG. 5Aobstructs the view of solder joint 150, the solder joint 152 of thepresent invention (Referring to FIG. 5B) can readily be seen at theperipheral edge of the package. Therefore, the resulting solder joint152 may be visually inspected for joint integrity. In some applications,the exposed solder joint 152 may also be more easily probed and testedsince the solder joint can be readily accessed from the side of thepackage. For a fixed sized lead frame, this permits the use of somewhatlarger solder joints which provides a greater potential mechanicalstrength to the joint due both to the increased area of attachmentbetween the contact 109 and the landing pad 158 of the printed circuitboard and to the increased volume of solder material in solder joint152. In other applications, this permits the use of smaller device areaswhile maintaining the same solder joint footprint, which may be used tofacilitate higher density lead frame panels.

Referring next to FIGS. 3-4, 5B, and 6B, a method of producing thedescribed packages will be explained. Generally, the lead frame panelsmay be formed and assembled using any appropriate process. By way ofexample, in a particular embodiment illustrated in FIG. 6B—a lead framepanel 180 is patterned to define a plurality of device areas and amatrix of tie bars. Each device area includes a multiplicity ofconductive contacts that are attached to an associated tie bar. Duringassembly, dice are attached to die attach pads 181 or otherwisepositioned within associated device areas (e.g., on a support tape ifdie attach pads are not provided). The dice are then electricallyconnected to the contacts (e.g., by wire bonding 182) and a casing ismolded 183 or otherwise provided that encapsulates the die andconnectors while leaving the bottom surfaces of the contacts exposed.

After the encapsulation has cured 184, portions of the contacts adjacentto the tie bars are undercut without severing the tie bars 185. By wayof example, the undercutting can be accomplished by a partial depthsawing operation along the tie bars. This undercutting exposes a sideand an underside surface of the contacts. The lead frame package is thenbuffed 186 and solder-plated 187 using industry standard techniques.After the lead frame package has been marked 188, tested 189, andsingulated 190 it is ready for shipping 191 or attachment to anelectronic component as described above. It should be apparent that theprimary difference between the present invention, and earlier processesis the addition of the Partial Cut 185. This step is generallyillustrated in FIG. 3.

In FIG. 3, a partial sawing operation is illustrated. In this operation,a relatively wide blade 131 is passed along the tie bar axis. The blade131 removes portions of the encapsulating molding (see above FIG. 2C125), the tie bar 111, and the tie bar stub 112 thereby exposing theside and underside surfaces of adjacent contacts 109. Cleaning themolding material from the contacts 109 and their corresponding side andunderside surfaces allows those surfaces to be solder-plated in asubsequent step. In the embodiment shown, a circular saw blade 131 isused, although it should be appreciated that any suitable technique maybe used to remove the molding material to expose the contact side andunderside surfaces including, but not limited to: grinding, etching,laser cutting, gouging, and other chemical and mechanical techniques.Furthermore, the partial sawing operation may be accomplished in singleor multiple operational steps.

In the described embodiment, the width of the blade 131 is slightlywider than the width of the molding material 125 as shown in FIG. 3,therefore cleaning the molding material from the contacts 109 and theircorresponding side and underside surfaces. The same result may beachieved using narrower blades in successive passes along the tie baraxes.

FIG. 4A illustrates the resultant undercut surfaces from the partial sawpass. The exposed undercut surfaces created by the partial saw pass maynow be solder-plated to facilitate attachment to printed circuit boardsor other electronic devices. Solder plating the exposed surfacesprevents surface oxidation of the contacts 109 that inhibits a reliablesolder connection to a printed circuit board or other electronic device.Solder plating 140 attaches to all exposed metallic surfaces includingthe adjacent contacts 109, the tie bar stub 112, the tie bar 111, andthe die attach pads 107. Solder plating does not attach to the moldingmaterial 125.

Once the contacts have been solder-plated, the lead frame panel is readyto be singulated or separated into individual devices. Referring to FIG.4B, the singulation cut is accomplished by conventional means. In thepresent embodiment shown, a singulation blade 141 is passed along eachtie bar axis on the lead frame panel. Singulation removes the tie bar111, a portion of the tie stub 112, and a portion of the moldingmaterial 125 leaving exposed molding and contact surfaces discussedbelow. It should be appreciated that the singulation blade 141 isgenerally narrower than the blade 131 described above for use in makingthe undercut. With this arrangement, a gap is created between the sideof the contact 109 and the edge of a package, which leaves the outsideedges of the contacts 109, as well as the undersides of the tie barstubs 112, exposed prior to solder plating. The ends of the tie barsstubs 112 are also exposed, however, they are not solder-plated.

The singulated packages may then be attached to a printed circuit boardor other appropriate substrate using standard attachment techniques(e.g., soldering). FIG. 5B illustrates a finished and singulatedleadless lead frame package mounted on a printed circuit board 153. Asseen therein, the solder joint 152 fills an area to the peripheral sideof the contact and under the tie bar stub 112 (which, in the versionillustrated in FIG. 5A, is occupied by the molding material). Thisprovides a strong, high quality joint that can be readily seen andaccessed from the side as described above. It should be apparent thatbecause the area of attachment of the contact 109 has been increased,the size of the landing pad 158 on the printed circuit board may need tobe increased a corresponding amount. The larger solder joint 152provides a more robust connection between the contact 109 and theprinted circuit board landing pad 158 as well as a visual inspectionpoint for solder joint integrity and a convenient test site that can bereached by conventional test probes over the prior art.

Referring next to FIG. 7A, another embodiment of the present inventionwill be described. FIG. 7A is a 3-dimensional perspective view of acontact 109 having a well 701 created as a result of etching the contactsurface 703. Before a die is attached to the lead frame panel as in Step1001 in FIG. 10, the lead frame panel is selectively etched at theintersection of the contact 109 and the tie bar 111. Etching the contact109 has the advantage of eliminating a Partial Cutting step as in Step185 in FIG. 6B. When the tie bars are etched to reduce their thickness,the simplest approach to forming the wells is to etch the wells at thesame time that the tie bars are being etched. It may be appreciated byone skilled in the art that etching may be accomplished by a variety ofmethods well known in the art.

FIG. 7B is a diagrammatic top view of a pair of contacts 109 having awell 701 as shown in FIG. 7A. As shown in FIG. 7B, the well 701 isaligned along the axis of the tie bar 111, which is part of a lead framepanel. The well 701 is sized such that the well 701 leaves an exposedregion on both contacts after the singulation cut. Singulation cut-lines702 demark the portions of the contact 109 and the tie bars 111 that areremoved during singulation of the panel. The side wall 705 of the well701 must be thick enough to withstand taping, encapsulating, and taperemoval without collapsing as well as narrow enough to provide areasonably sized well 701. Further, the leading ed ge 704 of the contact109 must be accordingly sized to resist collapse during singulationand/or other subsequent manufacturing.

In another embodiment, a singulation cut as demarked by the singulationcut-lines 702 leaves a portion of the exposed well continuous sidesurface 706 and the well bottom surface 707 of the contact 109 asillustrated in FIG. 7C—a diagrammatic cross sectional side view of anembodiment of the present invention. When the package is ultimatelyattach ed by soldering to an electronic device, the solder flows to theundercut portions of the contact formed by the well side surface 706 andthe well bottom surface 707 resulting in a stronger, more easilyinspected and tested joint.

In one particular described embodiment, the etching creates an exposedwell having an average depth of approximately 0.1 mm and an averagecircumference of approximately 0.3 mm. It is desirable in someembodiments to restrict the exposed well to within approximately 0.05 mmof the nearest side surface of the contact.

It should be appreciated that the shape of the wells may be widelyvaried. For example, FIGS. 8A-8C illustrate other suitable wellgeometries. FIG. 8A is a diagrammatic top view of a pair of contacts 109having an ovate shaped well 801. One advantage of this embodiment isthat the undercut region 805 of the contact after singulation, asdemarked by the singulation cut-lines 702, is larger than a similarlysized circle as illustrated in FIG. 7B. FIG. 8B, a diagrammatic top viewof a pair of contacts 109 having a substantially rectangular shaped well803 illustrates another embodiment of the present invention. Therectangular shaped well provides an even larger undercut region 806 thana similarly sized ovate well as illustrated in FIG. 8A with similaradvantages.

Another embodiment of the present invention is illustrated in FIG. 8C.FIG. 8C is a diagrammatic top view of a pair of contacts 109 havingcircular shaped wells 804. In this embodiment, the contacts 109 areconnected to the tie bar 111 by tie bar stubs 112. Here, a smaller well804 must be etched on each contact 109 to create an exposed region 807in the resulting semiconductor package. As can be appreciated by oneskilled in the art, a variety of well shapes may be achieved byselectively etching the lead frame surface depending on the particularmanufacturing requirements.

FIG. 9 is a diagrammatic cross sectional side view of a small section ofa lead frame panel in accordance with one embodiment of the presentinvention illustrating the exposed well 701 of the contact 109 protectedfrom encapsulation material 125 prior to a singulation cut as demarkedby singulation cut-lines 702. Prior to encapsulation, adhesive tape 801is adhered to the bottom surface 802 of the lead frame panel. Theadhesive tape 801 seals the well 701 thus preventing encapsulatingmaterial 125 from filling the void therein. The adhesive tape 801 alsoserves to ensure the encapsulating material 125 is substantiallyco-planer with the bottom surface 802 of the lead frame panel.

FIG. 10 is a flow chart illustrating a modified packaging process inaccordance with an embodiment of the present invention. Note inparticular, the first step 1000 wherein a lead frame pattern is providedwith etched wells. Etching is accomplished by any means common in theart to provide any of a number of shaped wells as described above. Itmay be appreciated that etching the lead frame pattern eliminates asubsequent Partial Cut step 185 as illustrated in FIG. 6B. As a furtheradvantage, well etching may be accomplished at the same time as the leadframe is etched to create device areas, contacts, and tie bars. Once thepattern has been provided, packaging of the device follows conventionalmanufacturing steps 1001-1010 as illustrated in FIG. 10.

Although only a few embodiments of the invention have been described indetail, it should be understood that the present invention might beembodied in many other specific forms without departing from the spiritor scope of the invention. For example, it should be apparent that thedescribed undercutting may be used with a wide variety of packagingprocesses and the application of the invention is not limited to theparticular packaging processes described.

As suggested, a variety of methods may be utilized to accomplish thepartial sawing. Further, the depth and width of the partial sawing maybe widely varied. By way of example, the initial cut may have an averagedepth of approximately 0.125 mm and an average width of approximatelygreater than 0.25 mm. Specific depths and widths of the partial saw aredependent on a particular application and are contemplated in thisapplication. In the primary embodiment described, a single sawing passusing a relatively wider blade is used to accomplish the undercutting.However, it should be appreciated that the same effect can be realizedusing multiple passes of a thinner blade. In the illustratedembodiments, narrower tie bar stubs 112 are used to couple the contactsto narrow tie bars 111. The narrowed tie bars and tie bar stubs tend tobe preferred to minimize the risk of shorting between contacts due tocopper (or other metal) streaking during sawing. However, the inventionmay also be used in embodiments where tie bar stubs are not used and/orthicker tie bars are used. In other applications, the tie bar stubs maybe the same width (or wider) than the contacts providing additionalsurfaces to which the solder can adhere.

Moreover, in another embodiment of the present invention the distancebetween the bottom surface portions of adjacent contacts in adjacentdevice areas are spaced to no more than approximately 0.45 mm. Spacingbetween the contacts is critical because of the partial cut operation.In particular, if the spacing is too narrow, the partial cut operationwill remove an excessive amount of contact material thus compromisingthe electronic and mechanical integrity of the contact. Alternatively,if the contact spacing is too wide, then the partial cut operation mayremove material only from the tie bar stubs rather than from thecontacts.

Additionally, the size, geometry and placement of the described wellsmay be widely varied without departing from the spirit of the invention.As suggested above, the wells can be circular, oval, rectangular,square, elongated or any appropriate geometry and the size of the sidewalls can be varied to meet the needs of a particular embodiment.

As suggested above, when it is known that the described process will beused, the lead frame panels may be designed to take advantage of thegreater strength solder bonds that are achievable. By way of example,the size of the contact pads that are co-planer with the bottom surfaceof the package can be reduced while maintaining overall joint strength.

The invention may be used in conjunction with any suitable conductivelead frame material. In present applications, copper and copper alloy-42are the most common lead frame materials, but the invention may be usedin conjunction with lead frames made from other materials, includingaluminum and other metals. A number of conventional package processingtechniques have been described as being used to accomplish specificsteps in the formation of the described devices. It should be apparentthat in most cases these processing techniques can be widely varied anda wide variety of alternative conventional processes may be used intheir place. Accordingly, the present examples are to be considered asillustrative and not restrictive, and the invention is not to be limitedto the details given herein, but may be modified within the scope of theappended claims.

1. A method of packaging semiconductor devices comprising: providing alead frame panel patterned to define a plurality of device areas and amatrix of tie bars having tie bar axes, each device area including amultiplicity of conductive contacts that are attached to an associatedtie bar; positioning a plurality of dice within the lead frame, each diebeing positioned within an associated device area; electricallyconnecting each die to the contacts of its associated device area usingconnectors; forming at least one casing over the device areas, therebyencapsulating the connectors and providing mechanical support for thecontacts while leaving the bottom surfaces of the contacts exposed;undercutting portions of the contacts adjacent the tie bars therebyexposing at least some side and underside surfaces of the contacts; andsolder plating the lead frame after the undercutting such that theexposed conductive contacts including the exposed side and undersidesurfaces are solder-plated.
 2. A method as recited in claim 1 furthercomprising singulating the device areas thereby providing singulatedpackaged semiconductor devices.
 3. A method as recited in claim 2,wherein the singulation is accomplished by severing along the tie bars.4. A method as recited in claim 2 wherein the undercutting isaccomplished by sawing along the tie bar axis using a first blade andthe singulation is accomplished by sawing along the tie bar axis using asecond blade that is narrower than the first blade, wherein theundercutting sawing operation does not sever the tie bars.
 5. A methodof packaging semiconductor devices comprising: providing a lead framepanel is patterned to define at least one two-dimensional matrix of tiebars that define a two dimensional array of immediately adjacent deviceareas that are separated only by the tie bars, each device areaincluding a multiplicity of conductive contacts that are attached to anassociated tie bar and wherein the contacts are patterned to include abase portion that includes the exposed bottom surface and stub portionsthat attach to the tie bars, the stub portions being thinner andnarrower than the base portions of their associated contacts;positioning a plurality of dice within the lead frame, each die beingpositioned within an associated device area; electrically connectingeach die to the contacts of its associated device area using connectors;forming at least one casing over the device areas, thereby encapsulatingthe connectors and providing mechanical support for the contacts whileleaving the bottom surfaces of the contacts exposed; undercuttingportions of the contacts adjacent the tie bars thereby exposing at leastsome side and underside surfaces of the contacts; and solder plating thelead frame after the undercutting such that the exposed conductivecontacts including the exposed side and underside surfaces aresolder-plated.
 6. A method as recited in claim 5 wherein theundercutting is accomplished by a first cutting operation along the tiebar axes, wherein the first cutting operation exposes the stubs and aside portion of the base of each contact, but does not sever the tiebars or the stubs, the method further comprising singulating the deviceareas after the solder plating using a second cutting operation alongthe tie bar axis.
 7. A method as recited in claim 6 wherein the firstcutting operation is performed using a first saw blade and the secondcutting operation is performed using a second saw blade that is narrowerthan the first blade.
 8. A method of packaging semiconductor devicescomprising: providing a lead frame panel patterned to define a pluralityof device areas and a matrix of tie bars having tie bar axes, eachdevice area including a multiplicity of conductive contacts that areattached to an associated tie bar wherein at least some of the contactshave wells formed therein; positioning a plurality of dice within thelead frame, each die being positioned within an associated device area;electrically connecting each die to the contacts of its associateddevice area using connectors; forming at least one casing over thedevice areas, thereby encapsulating the connectors and providingmechanical support for the contacts while leaving the bottom surfaces ofthe contacts exposed; solder plating the lead frame after the at leastone casing has been formed such that the exposed conductive contactsincluding the exposed wells are solder-plated.
 9. A method as recited inclaim 8 further comprising singulating the device areas therebyproviding singulated packaged semiconductor devices.
 10. A method asrecited in claim 9 wherein the singulation is accomplished by severingalong the tie bars.
 11. A method of packaging semiconductor devicescomprising: providing a lead frame panel patterned to define a pluralityof device areas and a two dimensional matrix of tie bars having tie baraxes each device area including a multiplicity of conductive contactsthat are attached to an associated tie bar wherein at least some of thecontacts have wells formed therein wherein the wells each include acontinuous side surface and a well bottom surface; positioning aplurality of dice within the lead frame, each die being positionedwithin an associated device area; electrically connecting each die tothe contacts of its associated device area using connectors; forming atleast one casing over the device areas using an encapsulant material,thereby encapsulating the connectors and providing mechanical supportfor the contacts while leaving the bottom surfaces of the contactsexposed, wherein the casing is formed with the wells in place, thegeometry of the contacts being arranged such that encapsulant does notenter the wells during the formation of the casing; and solder platingthe lead frame after the at least one casing has been formed such thatthe exposed conductive contacts including the exposed wells aresolder-plated.
 12. A method as recited in claim 11 wherein thecontinuous side surface is arcuate.
 13. A method as recited in claim 11wherein the continuous side surface is substantially rectangular.